Interruptors, Isolating Switches & CT/PT — Traction Distribution Notes

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220kV/25kV Breaker Technical Particulars — Completed

Particular	220kV	25kV
Rated short circuit breaking current	31.5 kA	20 kA
Rated breaking capacity (Two Pole)	7717.5 MVA	450 MVA
Rated breaking capacity (Three Pole)	13366.7 MVA	—
Rated making current	78.8 kA	50 kA
Rated operating sequence	0-0.3sec, CO-3sec-CO	0-0.3sec, 30sec-CO
Total breaking time	Not more than 80 ms	Not more than 65 ms
Rated short-time withstand current	31.5 kA for 1 second	20 kA for 3 seconds

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Exam Focus — 25kV Breaks FASTER Despite Being Lower Voltage

A genuinely counter-intuitive real detail: the 25kV breaker's total breaking time (65 ms max) is actually FASTER than the 220kV breaker's (80 ms max) — matching the earlier finding that 25kV also carries a higher rated normal current (1600A vs 1250A). Both facts reflect the same underlying reality: the lower-voltage secondary side handles closer-to-the-load conditions where speed and current capacity matter more directly.

25kV Interruptors — Real Specifications

Interruptors are provided at feeding posts and switching stations. Most interruptors are NOT required to clear faults — the ONE exception is the interruptor at the sectioning post, called a bridging interruptor, which IS called upon to clear faults under extended-feed conditions (directly matching the emergency-feed extension role covered in the earlier Substation Layout chapter).

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Exam Focus — Old vs New RDSO Interruptor Specs

The bridging interruptor's ORIGINAL breaking capacity was specified as 4000 Amps at 27.5kV recovery voltage, with a short-time current withstand capacity of 4000 Amps for 3 seconds. With higher-rating traction transformers now in use, RDSO's NEW specification has increased this to 8kA breaking capacity and 800 Amps normal rated current — a real, quantified upgrade worth distinguishing from the older figures.

Isolating Switches

To guard against mistakes while working on sub-station equipment, safety demands an apparatus that makes a VISIBLE break in the circuit — this is the isolating switch (isolator), also called an off-load switch since it opens/closes only when negligible or no current flows.

Primary (HV) side isolators: 2-pole or 3-pole. Constructionally divided into: (i) three-post, centre-post rotating double-break type, and (ii) two-post single-break type.
Primary-side isolators are HORIZONTAL break type; secondary (25kV) side isolators are VERTICAL break type — a real, testable orientation distinction between the two voltage levels.
Rated normal current of isolators is fixed to MATCH the rated current of circuit breakers and busbars — following the same IEC/IS standardised series covered in the earlier Breaker Selection Criteria chapter.

Current & Potential Transformers

Bushing-type CTs are provided on BOTH primary and secondary sides of traction transformers, exclusively meant for DIFFERENTIAL PROTECTION — directly connecting to the differential relay content covered in the earlier Transformer Protective Devices chapter (spill current from imperfect CT-secondary balance being one of the real mal-operation causes). Separately-mounted 132kV and 25kV CTs, plus 25kV PTs, are of conventional type.